This invention relates generally to image projection methods and apparatus for creating color or gray scale images using spatial light modulators (SLMs) without color or gray capability. This technique can be especially useful for displaying color and gray scale images in volumetric 3D displays. A volumetric 3D display displays 3D images in a real 3D space. Each “voxel” in a volumetric image locates actually and physically at the spatial position where it is supposed to be, and light rays travel directly from that position toward omni-directions to form a real image in the eyes of viewers. As a result, a volumetric display possesses all major elements in both physiological and psychological depth cues and allows 360° walk-around viewing by multiple viewers without the need of special glasses. For examples, [Tsao et al. 1998] and [Tsao 1999] disclose principles of volumetric 3D displays based on an approach that, using an optical interfacing mechanism, smoothly delivers whole frames of 2D images, which are created on an SLM and projected from a non-moving projector, to a moving screen and displays them on the screen. The motion of the screen effectively distributes the 2D image frames displayed on the screen in a space. These spatially distributed image frames form a volumetric 3D image in the eyes of viewers due to the after image effect of human eye. In addition, this invention is also useful for 2D display applications, such as projection of 2D images and high frame rate image display for optical correlators.
Many SLMs do not have color or gray scale capability. For example, liquid crystal displays (LCD) generally use the orientation of the liquid crystal molecules in the pixel cells to modulate the amount of light transmitting through or reflected from the pixels. LCD devices are therefore only capable of binary images (black and white) or gray scale images, but not color images. To display colors using LCDs, the most common practice is to build a color filter mosaic over the pixel cells, which is sometimes called a color triad or color stripe structure. Color images can then be displayed using these colored pixel cells. Using color filter mosaic to create color triad structure has the shortcoming of low optical throughput since only about ⅓ of a white light passes any one of the color filters. Spatio-chromatic colored illumination techniques using diffractive optics or holographic elements have been developed to provide high optical throughput. For example, [Joubert] describes an arrangement using a grating system to separate R, G and B colors on a micro-scale and then using a micro-lens array to focus the R, G and B spots to different pixel locations. Equivalently, the function of the grating system and the micro-lens array can be integrated by using a holographic micro-lens array. Alternatively, a micro-lens array with built-in gratings can also be used for this function, as described in [Morris]. These techniques therefore can provide a color triad pattern with high optical throughput.
Alternatively, color images can be displayed by a combination of projections from several LCDs, each illuminated by a different color. For example, three LCDs, each illuminated by a light beam of red, green and blue color, can produce images of various color combinations by projection. For LCDs with high frame rate, such as display devices based on ferroelectric liquid crystals (called FLCD), a field sequential technique can be used to create color images using only one display device with no color triads [Displaytech].
In addition to LCDs, there are other devices that are capable of only binary images or gray scales images. For example, digital micro-mirror device (DMD) consists a matrix of actuated small reflectors. By striking the matrix with a light beam and switching the position of each reflector, arbitrary images can be generated on the device and can be projected [Thompson & Demond]. For another example, thin-film micro-mirror array (TMA) also consists of a matrix of piezo-actuated micro-mirrors and each mirror can be actuated at multiple positions to create gray scale [Kim & Huang]. And there are also many other micro-mirror based devices.
When used as the image source for volumetric 3D displays or for high speed optical correlators, the display device must have high frame rate. For volumetric 3D display, higher frame rate gives higher number of frames per image volume, hence higher resolution in the direction of screen sweep. For optical correlators, higher frame rate give higher processing (correlation) throughput. In these applications, display devices of binary image capability can give only limited color/gray capacity using conventional color forming practices. Particularly, field sequential technique is not suitable because each single frame has to have color/gray. Three display devices (R, G and B each) can be used to provide colors, but more display devices means higher cost. Color triad techniques can be applied, but fabricating color filter mosaic over display devices is costly, especially many high frame rate devices, such as FLCD and DMD, are reflective-type display and have fine pixels with pitch on the order of 10 micro-meters. The previously mentioned spatio-chromatic illumination techniques are also difficult to apply to these reflective type displays. The gratings and micro-lens (or equivalent holographic optical elements) used to create color patterns are generally attached or placed close to the back of a transmissive type LCD. Building the diffractive optics on the reflective SLM or attaching them to the SLMs are not very desirable.
When color triad techniques are applied to generate 2D color images, artifacts such as color aliasing such as Moire patterns can appear, because R, G and B sub-pixels are not perfectly mixed in space.
It is therefore the purpose of the current invention to provide methods and optical setups that allows the use of single binary SLM to create color and gray images without the need of color triad optics built-in on the display device. It is also the purpose of this invention to increase color/gray capacity of volumetric 3D displays that use, especially but not limited to, binary SLM. In addition, this invention can be applied to increase the resolution of volumetric 3D display systems in the screen motion direction. This invention can also be applied to 2D projectors, using a single display panel with color triad structure, to project images without color triad artifacts.